When and why did several large population movements occur in history?

During the development of China, due to the constant wars and changes in political strategies, the population is constantly changing its residence. In ancient times, the social environment was complex, and the rulers could not deal with wars in time, which led to many people fleeing to other places in every war to protect their lives. Of course, they fled not only because of the war, but also to live and find a place to live and work in peace and contentment. However, the migration in history also paved the way for the development of the present society.

Every time a war broke out in ancient times, more or less people fled to other countries. Then the migration of these people has also created our good life now. There have been six major population movements in history, and the labor force of each movement will be much less, which is also beneficial to the resettlement areas.

One migration I still remember is the An Shi Rebellion. In the mid-Tang Dynasty, there was an Anshi rebellion in the north, and the Tang government troops fought against the enemy. At this time, at home and abroad in the Tang Dynasty, a large number of residents fled from the north to the south, mainly to Sichuan, and became shelters for the fleeing people. This has also led to a sudden surge in the number of local hukou, which has brought a great burden to people's lives.

Another big population migration was the Yongjia Rebellion, which can be said to be a relatively big war this time. This era is the most chaotic and cruel. The demise of the Western Jin Dynasty led to the demise of the whole country. Not only the country perished, but also the people were implicated. The enemy troops in the north burned, killed and looted, and treated the Han people as slaves. Many people fled to the south for their lives, and then the population in the north decreased sharply. ?